It is known in the art of medical radiography to employ intensifying screens to reduce the X-ray dosage to the patient. Intensifying screens absorb the X-ray radiation and emit electromagnetic radiation which can be better absorbed by silver halide emulsion layers. Another approach to reduce the X-ray dosage to the patient is to coat two silver halide emulsion layers on the opposite sides of a support to form a duplitized radiographic element. Accordingly, it is a common practice in medical radiography to use a radiographic assembly consisting of a duplitized radiographic element interposed between a pair of front and back screens.
The typical structure of an intensifying screen comprises a support and a phosphor layer coated thereon. The phosphor layer comprises a fluorescent substance able to emit light when exposed to X-ray, mixed with a binder. Additionally, a primer layer is sometimes provided between the phosphorescent layer and the substrate to assist in bonding the phosphorescent layer to the substrate, and a reflective layer is sometimes provided between the substrate (or the primer) and the phosphorescent layer. Finally, a protective layer for physically and chemically protecting the screen is usually provided on the surface of the phosphorescent layer.
Intensifying screens are usually prepared by preparing a dispersion of the phosphor/binder mixture in an appropriate solvent and coating such a dispersion on a support using any well-known coating method (e.g., doctor blade, roll coater, knife coater and the like). After the coating, the solvent is removed from the phosphor layer.
The screens most widely used in radiography can be classified according their characteristic speed in (1) fast screens, (2) average screens, and (3) slow screens. The speed of a screen is directly proportional to the thickness of the phosphor layer, that is, to the amount of phosphor in the phosphor layer, and to the grain size of the phosphorescent substance.
A well known problem of intensifying screens relates to the sharpness of the resulting image. The presence of the intensifying screen reduces the sharpness of the resulting image, in particular when fast screens are used. This is due to the large phosphorescent crystals and to the high thickness employed to realize the fast screens. However, there are situations where a reduction in exposure is of prime importance in spite of some sacrifice in image sharpness.
Another critical problem of intensifying screens relates to the noise or granularity of the resulting image. Granularity of intensifying screens can be improved by increasing the amount of phosphor in the phosphor layer, but this negatively affects sharpness. Sharpness and granularity concurs together to define the quality of the resulting radiographic image.
In recent years there have been many attempts to produce radiographic screens having an improved relationship between speed and image quality (sharpness and granularity).
U.S. Pat. Nos. 4,952,813 and 4,910,407 disclose a method for preparing an intensifying screen in which the phosphor contained in a phosphor layer is densely packed by means of a compression treatment which reduces the percentage of voids in the phosphor layer. The method disclosed in U.S. Pat. No. 4,952,813 can improve the sharpness of the resulting intensifying screen, but speed and granularity are significantly reduced. U.S. Pat. Nos. 5,164,224 and 5,153,078 disclose a method for preparing radiation image storage and intensifying screens similar to U.S. Pat. No. 4,952,813, but using a specific class of thermoplastic binder which is capable of reducing the deterioration of the speed and graininess caused by the compression treatment. The sharpness of the screens obtained with the teaching of the above mentioned patents is not, however, significantly improved. EP 579,016 and EP 577,027 teach the use of a specific fluorinated top-coat having a thickness lower than 5 .mu.m in order to achieve a further improvement in residual stain. However, the use of the specific fluorinated top-coat has no particular effect on sharpness, speed and graininess of the resulting intensifying screen. The compression treatment disclosed in these patents provides a higher packing density of the phosphor in the phosphor layer, but also causes a mechanical stress to the phosphor particles, reducing their efficiency and, consequently, speed, and requires an additional and expensive compression step during manufacturing.
U.S. Pat. No. 4,292,107 discloses an intensifying screen in which a UV-radiation curable resin is used as a binder for the phosphor layer. The preparation method consists of (1) preparing the dispersion of the phosphor in a UV-curable binder consisting of an unsaturated resin, a polymerizable monomer and a sensitizer with a phosphor to binder ratio of from 1:1 to 20:1, (2) coating the resulting dispersion on a polyethylene terephthalate film support to provide a phosphor layer, and then (3) curing the resulting phosphor layer. Unsaturated resins useful in the process of this patent are defined as polyester acrylate, urethane polyester acrylate, polyester methacrylate, epoxy acrylate, and polyether acrylate.
U.S. Pat. Nos. 5,411,806 and 5,520,965 disclose a method for producing an intensifying screen with better speed/sharpness relationship by employing a curable composition containing less than 5% of non-curable organic material having molecular weight less than 500. The preferred curable composition disclosed in these patents comprises UV curable acrylamidosiloxane polymers as described in U.S. Pat. No. 5,091,483.
In spite of the above-mentioned work, there is still the need to have an intensifying screen which shows high speed with high image quality, in particular improved granularity.